Reconstitution of CO2regulation of SLAC1 anion channel and function of CO2-permeable PIP2;1 aquaporin as carbonic anhydrase 4 interactor

ABSTRACT Daily dark periods cause an increase in the leaf CO 2 concentration ( Ci ) and the continuing atmospheric [CO 2 ] rise also increases Ci . Elevated Ci causes closing of stomatal pores thus regulating gas exchange of plants. The molecular signaling mechanisms leading to CO 2 -induced stomatal closure are only partially understood. Here we demonstrate that high intracellular enhances currents mediated by the guard cell S-type anion channel SLAC1 when co-expressing either of the protein kinases OST1, CPK6 or CPK23 in Xenopus oocytes. Split-ubiquitin screening identified the PIP2;1 aquaporin as an interactor of the β CA4 carbonic anhydrase, which was confirmed in split luciferase, bimolecular fluorescence complementation and co-immunoprecipitation experiments. PIP2;1 exhibited CO 2 permeability. Co-expression of β CA4 and PIP2;1 with OST1-SLAC1 or CPK6/23-SLAC1 enabled extracellular CO 2 enhancement of SLAC1 anion channel activity. An inactive PIP2;1 point mutation was identified which abrogated water and CO 2 permeability and extracellular CO 2 regulation of SLAC1 activity in Xenopus oocytes. These findings identify the CO 2 -permeable PIP2;1 aquaporin as key interactor of carbonic anhydrases, show functional reconstitution of extracellular CO 2 signaling to ion channel regulation and implicate SLAC1 as a bicarbonate-responsive protein in CO 2 regulation of S-type anion channels.